1. Field of the Invention
The invention generally relates to dense wavelength division multiplexers (DWDM) and in particular to a technique for determining transmission wavelengths of lasers of the DWDM as a function of laser control tuning parameters.
2. Description of the Related Art
A DWDM is a device for simultaneously transmitting a set of discrete information channels over a single fiber optic transmission line. A conventional fiber optic transmission line is capable of reliably transmitting signals within a bandwidth of 1280 to 1625 nanometers (nm), the xe2x80x9clow lossxe2x80x9d region for silica fiber. Within that overall bandwidth, the International Telecommunications Union (ITU) has defined various transmission bands and specified certain transmission channel protocols for use within each transmission band. One example of a transmission band is the ITU xe2x80x9cCxe2x80x9d band, which extends 40 nm from 1525 nm to 1565 nm. Within the C band, specific transmission channel protocols of 40, 80, or 160 discrete channels are defined and, for each protocol, the ITU has defined a grid of transmission wavelengths, with each line corresponding to an acceptable transmission wavelength. For the 40 channel protocol, the corresponding ITU grid has 40 lines with channel spacing of 0.8 nm; for the 80 channel protocol, the corresponding ITU grid has 80 lines with channel spacing of 0.4 nm; and so forth. The protocols have been defined to ensure that all DWDM transmission and reception equipment are fabricated to operate at the same wavelengths. Other exemplary ITU transmission bands are the S- and L-bands.
To simultaneously transmit the set of channels on a fiber optic cable, the DWDM employs a set of individual distributed feedback (DFB) lasers, with one DFB laser per channel. FIG. 1 illustrates a DWDM 100 having forty individual DFB lasers 102 for transmitting optical signals via a single optic fiber 104. An optical multiplexer 106 couples signals received from the individual DFBs via a set of intermediate optic fibers 107 into output optic fiber 104. Each DFB laser transmits at a different wavelength of the forty channel ITU C band. This enables forty separate channels of information to be transmitted via the single optical fiber 104 to a de-multiplexer (not shown) provided at the far end of the optical fiber.
To permit the DWDM to transmit forty separate channels simultaneously, each individual DFB must be tuned to a single ITU transmission channel wavelength. A DFB laser can be tuned only within a narrow wavelength band, typically about 4 nm in width. Hence, for the 40 channel protocol of the ITU C band having 0.8 nm transmission line spacing, the typical DFB can only be tuned to one of a few adjacent lines out of the total of 40 lines of the ITU grid. Traditionally, each individual DFB laser is manually calibrated at the factory to emit at one of the corresponding ITU transmission lines. This calibration is achieved by adjusting the laser operating temperature and current to obtain the desired wavelength. The laser is then, in some implementations, locked to the target wavelength by routing the output beam from each DFB laser through a corresponding manually-tunable etalon. (The etalons are not shown in FIG. 1.) A manually-tunable etalon is an optical device which produces a periodically-varying transmission spectrum as a function of laser wavelength. By tilting the etalon relative to the DFB laser beam path, a transmission peak of the etalon can be made coincident with the target ITU channel. The wavelength of an etalon transmission peak is calibrated to one of the ITU transmission lines by manually adjusting the angle of the etalon while monitoring the wavelength output from the etalon using an optical wavelength analyzer. The angle of the etalon is adjusted until the output wavelength is properly aligned with one of the ITU transmission lines, then the etalon is mounted in place in an attempt to lock the output wavelength of etalon to the selected ITU transmission line. This is a difficult and time consuming process requiring skilled technicians. Calibration of all forty DFB lasers of a single DWDM can be quite expensive. Mechanical or thermal drift of the etalon over time often moves the transmission peak away from the target ITU channel which requires recalibration.
Once the DFB lasers of a single DWDM are properly aligned with the ITU grid, the DWDM may then be used for transmitting signals over a fiber optic line, such as for transmitting digital data over computer networks (i.e., the Internet) or for transmitting television signals from a television network to one of its affiliates. A single DWDM must be provided for use with each fiber optic line employed for DWDM transmissions and hence a single customer installation, such as a television broadcast center, may require many, many DWDMs. If one of the DFB lasers within a DWDM drifts from its corresponding ITU transmission line or otherwise malfunctions, the entire DWDM typically needs to be replaced requiring the malfunctioning DWDM to be returned to the factory to be re-calibrated or otherwise fixed. As a result, the cost of maintaining a set of DWDMs is often substantial. To help remedy this problem, some DWDMs are provided with an additional widely tunable laser (WTL) which can be tuned separately to any one of the ITU grid lines. Hence, if one of the DFB lasers malfunctions, the single WTL can be tuned to the corresponding transmission wavelength of the DFB to thereby permit the DWDM to continue to operate. Additional WTLs can be supplied with a DWDM to accommodate the failure of two or more DFB channels, and such xe2x80x9csparingxe2x80x9d is a major advantage a WTL over a DFB. However, the WTL cannot simply and accurately be tuned to any target ITU channel at a customer installation and must be calibrated at the factory for operation at a specific channel.
Another problem associated with employing DFB lasers within DWDMs is that, because each DFB laser can only be tuned within a narrow range of about 4 nm, each DFB laser can only be calibrated to one of a few adjacent ITU transmission wavelength lines. It may also sometimes be desirable to configure the DWDM to use many lasers for transmitting at a single ITU transmission line to provide more bandwidth on that channel. When using DFB lasers, no more than two or three of the lasers can be calibrated to a single ITU transmission line. Hence, in some DWDMs, WTLs are used exclusively instead of DFB lasers, thus permitting any of the lasers to be manually calibrated at the customers installation to transmit on any of the ITU transmission lines. Although the use of WTLs remedies many of the problems associated with using DFB lasers, WTLs are difficult and expensive to fabricate and initially calibrate, and are susceptible to wavelength drift requiring frequent recalibration at the customers installation by trained technicians and hence necessitating high overall installation and maintenance costs.
Thus, whether using DFB lasers or WTLs within a DWDM, significant problems arise in achieving and maintaining proper wavelength calibration of the lasers to permit reliable operation of the DWDM. Accordingly it would be desirable to provide an efficient method and apparatus for calibrating transmission lasers within a DWDM and it is to that end that the invention is primarily directed.
In accordance with a first aspect of the invention, a method and apparatus is provided for calibrating a laser using an etalon and a gas absorption cell containing a gas of known light absorption characteristics. In accordance with the method, an output beam from the laser is routed through the etalon while the laser is tuned through a range of tuning parameters to produce an etalon transmission spectrum as a function of the laser tuning parameters. The output beam from the laser is also routed through the gas cell while the laser is tuned through the range of tuning parameters to produce a gas absorption spectrum as a function of the laser tuning parameters. The etalon transmission spectrum and the gas absorption spectrum are detected and then compared to determine the absolute transmission wavelength of the laser as a function of the laser tuning parameters. Thereafter, the laser can be set to transmit at a selected transmission wavelength by using the tuning parameters that correspond to the selected wavelength.
In an exemplary embodiment, the invention is implemented as a hand-held wavelength mapper for use with a transmission WTL of a DWDM to be tuned to an ITU transmission grid line for transmission through an optic fiber. The laser is tuned using a single tuning parameter, which may be an input voltage or current. The known gas, which may be hydrogen cyanide, acetylene or carbon dioxide, is contained within a sealed gas cell of the wavelength mapper. A portion of the output beam of the laser is split off and routed separately through the etalon and the gas absorption cell to two separate detectors for detecting both an etalon transmission spectrum and a gas absorption spectrum.
The absolute transmission wavelengths of the WTL as a function of the input voltage or current WTL tuning parameters are determined as follows. The detected etalon transmission spectrum has transmission peaks that are separated by a precisely known wavenumber (wavenumber is the number of wavelengths of laser light per cm, and so is inversely proportional to wavelength) determined by the construction material, physical dimension, and optical properties of the etalon. This wavenumber xe2x80x9ccombxe2x80x9d is exploited to determine relative wavenumbers for the tuning parameters. To this end, transmission lines in the detected etalon transmission spectrum are identified, a relative wavenumber is assigned to the tuning parameter corresponding to each consecutive etalon transmission line, and then relative wavenumbers are assigned to each intermediate value of the tuning parameters by interpolating between the transmission lines. Next, the detected gas absorption spectrum, which is a function of WTL tuning parameters, is converted to a modified gas absorption spectrum, which is a function of relative wavenumber, by assigning relative wavenumbers to each value of the detected gas absorption spectrum based on the associated tuning parameter. Then, the modified gas absorption spectrum is compared with an input gas absorption spectrum, which is a function of absolute wavenumber, to determine corresponding absolute wavenumbers for each value of the tuning parameters. This is achieved by inputting a predetermined gas absorption spectrum specifying absorption as a function of absolute wavenumber; correlating the modified gas absorption spectrum, which is a function of relative wavenumber, with the input absorption spectrum, which is a function of absolute wavenumber, to determine an offset between relative wavenumbers and the absolute wavenumbers; and then adjusting the relative wavenumbers associated with each value of the tuning parameters by the offset value to provide an absolute wavenumber for each value of the tuning parameters. If needed, the wavenumbers can be easily converted to wavelengths or frequencies. In this manner, the absolute transmission wavelength, frequency, or wavenumber of the WTL is thereby determined as a function of the tuning parameters.
Hence, by tuning the output wavelength of the WTL and using an etalon in combination with a gas absorption cell, the WTL can be quickly, easily and precisely set to a selected ITU transmission grid line at a customers installation. The tuning process can be periodically repeated to maintain precise tuning of the WTL despite possible temperature or mechanical drift. Thus overall installation and maintenance costs associated with DWDMs can be significantly reduced. By providing precise and reliable tuning of the lasers of the DWDM, the invention also facilitates the use of a greater number of transmission channels, such as 80, 160 channels, or more.
In general, any laser tunable using any set of input tuning parameters, such as various combinations of input analog or digital signals, can be used with the invention so long as an appropriate gas absorption reference is available. The laser is simply scanned through its full range of tuning parameters to enable determination of the absolute output wavelength of the laser as a function of any combination of the tuning parameters.
In an alternative embodiment, particularly for use with lasers employing two or more tuning parameters, a wavelength mapping technique is provided using a pair of etalonsxe2x80x94one comparatively thick and one comparatively thin. The thin etalon is employed to produce a fairly coarse relative wavelength map covering an entire transmission band of interest, such as the entire ITU C-Band. The thick etalon is then employed in combination with a gas absorption cell to more finely map the wavelengths of the laser to its tuning parameters. Initially, an output beam from the laser is routed through the thin etalon while tuning the laser through a range of a first tuning parameter to produce a first coarse etalon transmission spectrum, which is a function of the first laser tuning parameter. Next, the output beam from the laser is again routed through the first etalon while tuning the laser through a range of a second tuning parameter to produce a second coarse etalon transmission spectrum, which is a function of the second laser tuning parameter. The two coarse etalon transmission spectra are combined to produce a two-dimensional map of relative wavelengths as a function of the tuning parameters. A microcontroller processes the two-dimensional map to determine combinations of tuning parameters for use in tuning the laser from a minimum output wavelength to a maximum output wavelength. The microcontroller then controls the laser to sweep from the minimum wavelength to the maximum wavelength while routing the output laser beam through the thick etalon and through a gas absorption cell. Using techniques described above, the microcontroller compares the resulting gas absorption spectrum and etalon transmission spectrum with a reference gas absorption spectrum to map the laser tuning parameters to absolute transmission wavelengths of the laser. In this manner, wavelength mapping is achieved for use with lasers requiring multiple tuning parameters in circumstances where a single etalon might not cover the entire transmission band of interest with sufficient precision.
In accordance with a second aspect of the invention, a method and apparatus is provided for locking a laser to a transmission wavelength using an etalon and a gas absorption cell. The etalon used to map the output wavelengths of the laser is a temperature-controlled etalon. The aforementioned wavelength mapping steps are performed to determine the absolute wavelength of the laser as a function of the laser tuning parameters. Tuning parameters are applied to the laser to tune the laser to a selected transmission wavelength, such as an ITU channel wavelength. Additionally, a temperature offset is applied to the etalon to vary the wavelengths of the transmission peaks of the etalon until one of the transmission peaks is precisely aligned with the selected wavelength. Any drift of the laser output from the selected wavelength is detected and the tuning parameters applied to the laser are automatically adjusted to compensate for the drift. Thus, a feedback loop is provided which keeps the main output beam locked on a selected transmission channel despite possible variations in the output characteristics of the laser. In an exemplary embodiment, the invention is implemented as a wavelength locker for mounting to a transmission WTL of a DWDM to be tuned to an ITU transmission grid line for transmission through an optic fiber. In one specific implementation, the temperature offset is applied to the etalon by applying an electrical current to the etalon.